In a single-injection fuel injection system in which a single or uninterrupted charge of fuel is injected into an engine combustion chamber or cylinder during each combustion cycle, incomplete combustion can occur wherein the spray of fuel into the combustion chamber is not completely burned prior to the exhaust phase of the engine. Incomplete combustion can cause a large increase in engine emissions, and is therefore to be avoided as much as possible.
A different problem encountered during operation of an internal combustion engine (especially those using diesel fuels) is an excessive rate of pressure rise in the combustion chamber after ignition of the fuel charge. This condition is caused by a delay period from the beginning of injection of fuel to the initiation of ignition of the fuel. See, Burman, P. G. and DeLuca, F., Fuel Injection and Controls (1962), Chapter 11, "Diesel Engine Combustion" at p. 138. This excessive rate of pressure rise is undesirable because it applies high stresses to engine components and causes rough engine operation.
One known method of minimizing incomplete combustion and the excessive rate of pressure rise is known as "split injection." Split injection involves the injection of a pilot fuel charge into the combustion chamber prior to injection of a main or primary fuel charge during each fuel injection cycle. The pilot fuel charge is smaller in volume and duration of delivery than the main fuel charge and is ignited before the main fuel charge is injected into the combustion chamber. Injection of the pilot fuel charge results in burning of the main fuel charge in a relatively uniform and complete manner in comparison to a single-injection system, yielding smoother engine performance, reduced emissions, and reduced engine noise. The excessive rate of pressure rise in the combustion chamber and excessive emissions can be further controlled by controlling the fuel injected during the pilot injection.
U.S. Pat. No. 3,014,466 issued to Monnot et al. on Dec. 26, 1961, U.S. Pat. No. 3,216,407 issued to Eyzat on Nov. 9, 1965, and U.S. Pat. No. 4,022,165 issued to Eckert et al. on May 10, 1977, disclose the use of auxiliary fuel lines which permit injection of a pilot charge. The drawback of such a design is that it requires additional fuel lines. Also, installation of such a system in many applications may be difficult because of space constraints within the engine compartment.
Fuel injectors are known in the art which are designed to inject a pilot fuel charge without the need for other devices. U.S. Pat. No. 2,813,752 issued to Pringham on Nov. 19, 1957, U.S. Pat. No. 2,951,643 issued to Engel, Jr. on Sep. 6, 1960, and U.S. Pat. No. 3,104,817 issued to Vander Zee et al. on Sep. 24, 1963, disclose fuel injection nozzles having the capability to introduce a pilot charge and a main charge. However, these nozzles must typically be customized for each engine in which they are to be installed.
A paper authored by K. P. Mayer, entitled "Fuel economy, emissions and noise of multi-spray light duty DI diesels--current status and development trends," SAE Paper No. 841288 (1984), discloses the use of a separate mechanical split injection device disposed between a high pressure fuel line and a fuel injection nozzle. A piston loaded by a spring is disposed in fluid communication with the high pressure fuel line. As fuel pressure increases during each injection cycle, the piston retracts, thereby increasing the fuel volume. The increase in fuel volume results in a momentary reduction in fuel pressure delivered to the nozzle during each injection cycle. The Mayer paper does not disclose means for adjusting in situ the Mayer device for different applications or operating conditions.
In addition to the foregoing, U.S. Pat. No. 3,575,146 issued to Creighton et al. on Apr. 20, 1971, and U.S. Pat. No. 4,621,599 issued to Igashira et al. on Nov. 11, 1986, disclose electronically controlled fuel injection systems which regulate the flow of fuel during a pilot charge and a main charge. Igashira '599 also discloses a fuel injection control system which injects a third fuel charge in addition to main and pilot charges. These control systems are complex and add considerable cost to the design and manufacture of fuel injection systems. Furthermore, they are not readily retrofitted to existing engines which use conventional single-injection fuel systems.
U.S. Pat. No. 3,438,359 issued to Thoma on Apr. 15, 1969, discloses the use of a valve disposed between a high pressure fuel line and an injector nozzle. The valve regulates and meters the fuel flow such that a pilot charge is injected without the need for a specially adapted nozzle, separate pilot fuel line, or complex fuel injection control system.
The Thoma '359 device includes a piston disposed in a valve body which is preloaded by a helical compression spring.
At the beginning of each fuel injection sequence, the rise in fuel pressure which accompanies each injection pulse acts in opposition to the spring. Thus the piston is displaced and fuel flows into a reservoir. The flow of fuel into the reservoir causes a momentary pressure drop which defines the intermediate drop in fuel pressure between the pilot injection pulse and main injection pulse.
The disadvantages of the Thoma '359 device are twofold. First, the device is costly to produce because the intricate design of the piston demands a number of relatively expensive machining operations. Second, the device is not adjustable, and must be custom designed and built for each different engine design with which it is to be used.
Newer engines often include split injection capabilities. However, there is a need for an inexpensive, simple and reliable retrofit device for an existing engine having a single-injection fuel injection system to convert such an engine into one having split injection capability and which can facilitate precise control over fuel injection characteristics in order to control the rate of pressure rise in the combustion chamber and minimize incomplete combustion.